Radeon 9800 ......crashing my PC!!!

Hello, i've posted before about this problem and got some great advice but unfortunately the problem keeps rearing its ugly head.
When playing any game(Unreal, Joint Ops, C & C etc) my PC either freezes or the monitor says "no signal input", all the lights etc stay on on my tower but the PC shuts down, this happens after 2-5 minutes of gaming. It seems to happen more with Joint Ops than with other games???
I've tried taking the side off of my PC and blasting a desk fan into it to elimanate an overheating problem. I've also disabled/enabled fast writes and changed the AGP speed from 8x to 4x and back again in BIOS but to no avail. I've checked dxdiag and everything is coolio in there too.
i've read just about every remotely related thread in these forums trying to find out any clues at all.
If anyone can shine a little more light on this problem then your help will be much appreciated. Cheers in advance.
Oh yeah, almost forgot, i've downloaded the latest ATI 4.12 drivers and that didn't help either.